This invention relates in general to electrophotography and, more specifically, to a novel electrophotographic imaging member and process for using the imaging member.
In the art of electrophotography, an electrophotographic imaging member containing a photoconductive layer is imaged by first uniformly electrostatically charging the imaging surface of the imaging member. The member is then exposed to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the photoconductive layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided properly charged toner particles on the surface of the photoconductive layer to form a toner image which is thereafter transferred to a receiving member and fixed thereto.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite of layers containing a photoconductor and another material. One type of composite photoconductive photoreceptor used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Such a photoconductive layer is often referred to as a charge generating or photogenerating layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may function as a cathode when the charge transport layer is sandwiched between the electrode and a photoconductive layer which is capable of photogenerating holes and electrons and injecting the photogenerated holes into the charge transport layer. The transport layer in this embodiment must, of course, be capable of supporting the injection of the photogenerated holes from the photoconductive layer and transporting the holes through the transport layer to the conductive substrate when the outer surface of the photoconductive layer is charged with uniform charges of a positive polarity.
Other types of composite photoconductor employed in xerography include photoresponsive devices in which a conductive substrate or electrode is coated with optional blocking and/or adhesive layers, a charge transport layer such as a hole transport layer, and a photoconductive layer. Where the transport layer is a hole transport layer, the outer surface of the photoconductive layer is charged positively. These types of composite photoconductors are described, for example, in copending applications U.S. Ser. No. 613,137, filed on May 23, 1984, entitled "Silylated Compositions and Deuterated Hydroxyl Squaraine Compositions and Processes" and U.S. Ser. No. 487,953, filed on Apr. 25, 1983, entitled "Overcoated Photoresponsive Devices", the entire disclosures thereof being incorporated herein in their entirety.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain hole transporting aromatic amine compounds. Various charge generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical inorganic photoconductive materials utilized in the charge generating layer include amorphous selenium, amorphous silicon, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and the like. The organic photoconductive materials utilized in the charge generating layer include metal free phthalocyanines, metal phthalocyanines such as vanadyl phthalocyanines, substituted and unsubstituted squaraine compounds, thiopyrylium compounds and azo dyes, diazo dyes, pyrilium derivatives, and the like. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Some examples of homogeneous and particulate photoconductive materials with or without a polymeric binder in a charge generation layer are disclosed in U.S. Pat. No. 4,265,990, the disclosure of this patent being incorporated herein in its entirety.
Electrophotographic imaging members comprising a charge transport layer sandwiched between a photogenerating layer containing a vanadyl phthalocyanine pigment and a conductive substrate normally exhibit very good electrical properties. However, it has been found that when the photogenerating layer is formed by conventional solution coating techniques such as by means of a Bird (draw bar) or a dip coating device, the resulting photoreceptor often exhibits very poor electrical properties such as poor charge acceptance, premature charge injection and high dark decay rates. Dark decay is defined as the loss of charge on a photoreceptor in the dark after uniform charging. This is an undesirable fatigue-like problem resulting in lower initial charges that cannot be maintained during image cycling and is unacceptable for automatic electrophotographic copiers, duplicators and printers which require precise, stable, and a predictable photoreceptor operating range. In relatively adverse situations, the charging capability of the photoreceptor gradually decreases upon cycling (cycle-down) and the photoreceptor becomes unsuitable for copying and printing. In more favorable situations, the photoreceptor may experience low charge acceptance rates in the first few imaging cycles. The charge acceptance level gradually increases (cycle-up) upon cycling and eventually reaches an almost constant value. The poor initial charge acceptance of a photoreceptor causes poor image quality, light density, poor solid area density or image deletion in the first few xerographic copies. This problem becomes more serious if the photoreceptor has been used for some time and dark-rested for several hours (e.g. overnight). For example, the charge acceptance level after dark-resting for photoreceptors containing vanadyl phthalocyanine in the photoconductive layer (often referred to as a photogenerating or generator layer) is usually lower than it normally would be under conditions where it has not dark-rested for several hours. This problem causes poor image quality in printed copies. This condition is partially due to the premature injection of charges into the hole transport layer either from the surface of the photoreceptor or from the charge generation layer under the influence of an electric field. If machine adjustments to compensate for these changing properties are made, copies made layer during cycling exhibit high background. This difference in performance may be due to process variations and different impurities levels in the photogeneration layer. Since the use of conventional coating technology to prepare photoreceptors of these types is partially desirable for large flexible electrophotographic imaging members, there is a need to overcome these poor electrical properties associated with solution coating of the photoreceptor layers.
Moreover, when the supporting conductive layer in photosensitive members comprising at least two electrically operative layers has a metal oxide, difficulties have been encountered with these photosensitive members under extended electrostatic cycling conditions found in high volume high speed copiers, duplicators and printers. For example, when certain charge generation layers comprising a resin and particulate photoconductor material are adjacent to an aluminum oxide layer, the phenomenon of cycling up is encountered. Cycling up is the build up of residual potential through repeated electrophotographic cycling. Build-up of residual potential can gradually increase under extended cycling. Residual potential causes the surface potential to increase accordingly. Build-up of residual potential and surface voltage causes ghosting, increased background on final copies and cannot be tolerated in high speed, high-volume copiers, duplicators and printers. It has also been found that some photoreceptors comprising at least two electrically operative layers exhibit cycling down of the surface voltage when exposed extended cycling. During cycling-down, the surface voltage and surface charge decrease as dark decay increases in the areas exposed and the contrast potential for good images degrade and causes faded images. These problems have been addressed by the use of a siloxane film as described in U.S. Pat. No. 4,464,450. Although excellent results have been achieved with this siloxane film, "white spots" deficient of toner material are occasionally observed in image areas of final copies.
Thus, the characteristics of photosensitive members comprising a conductive layer and at least two electrically operative layers, one of which is a charge transport layer comprising a film forming resin and one or more aromatic amine compounds and hydrazones, can exhibit deficiencies which are undesirable in modern copiers, duplicators, and printers. Accordingly, there is a need for compositions and processes which impart greater stability to electrophotographic imaging systems which undergo periodic cycling.